Is Happy Students easy or hard?

Happy Students is rated Medium difficulty. The challenge is recognizing that only boundary positions in the sorted array can produce valid group sizes. Once that insight is clear, the implementation becomes straightforward.

Happy Students Python/Java solution

Most implementations follow the same pattern: sort the array, iterate through possible cut points, and check boundary conditions. The logic is identical across Python, Java, C++, and JavaScript, with differences only in syntax and built-in sorting functions.

How to solve Happy Students in O(n)?

A strict O(n) solution is generally not used because the ordering of values matters. Sorting simplifies the constraints and leads to a clean O(n log n) solution. After sorting, checking candidate group sizes only requires a single pass through the array.

What is the best approach for Happy Students?

The most common approach is sorting the array and evaluating valid cut points. After sorting, check each possible group size k and verify boundary conditions: nums[k-1] k (with edge cases for k=0 and k=n). This runs in O(n log n) time due to sorting and O(1) extra space.

Is Happy Students asked at Google/Amazon/Meta?

Problems involving sorting with boundary conditions and counting valid configurations are common in interviews at companies like Google, Amazon, and Meta. While the exact question may vary, the pattern of sorting and validating constraints appears frequently in coding interviews.

What data structure is used in Happy Students?

The problem primarily uses arrays along with sorting. Some variants of the solution also rely on binary search to count how many elements are less than a given threshold when testing candidate group sizes.

What is the time complexity of Happy Students?

The optimal solution runs in O(n log n) time because the array must be sorted first. After sorting, a single linear scan checks valid boundaries in O(n) time. Space complexity is O(1) if the sort is in-place.

Description Hints 3 Solution Video 11:16

2860. Happy Students

Medium50.9% acceptance1 company

You are given a 0-indexed integer array nums of length n where n is the total number of students in the class. The class teacher tries to select a group of students so that all the students remain happy.

The i^th student will become happy if one of these two conditions is met:

The student is selected and the total number of selected students is strictly greater than nums[i].
The student is not selected and the total number of selected students is strictly less than nums[i].

Return the number of ways to select a group of students so that everyone remains happy.

Example 1:

Input: nums = [1,1]
Output: 2
Explanation: 
The two possible ways are:
The class teacher selects no student.
The class teacher selects both students to form the group. 
If the class teacher selects just one student to form a group then the both students will not be happy. Therefore, there are only two possible ways.

Example 2:

Input: nums = [6,0,3,3,6,7,2,7]
Output: 3
Explanation: 
The three possible ways are:
The class teacher selects the student with index = 1 to form the group.
The class teacher selects the students with index = 1, 2, 3, 6 to form the group.
The class teacher selects all the students to form the group.

Constraints:

1 <= nums.length <= 10⁵
0 <= nums[i] < nums.length

Asked at 1 company

Amazon

Topics

Hints open one at a time

Hint 1

If a student with nums[i] = x is selected, all the students with nums[j] <= x must be selected.

Hint 2

If a student with nums[i] = x is not selected, all the students with nums[j] >= x must not be selected.

Hint 3

Sort values in nums and try all possible values for x from 0 to n separately.

11:16

Still stuck?Read 3 approaches, or watch a walkthrough.

Open solution

LanguageLoading editor…

1class Solution:
2    def countWays(self, nums: List[int]) -> int:
3        

Starter codePython 3Spaces: 4

Examples

Case 1Case 2

Input

[1,1]

Input

[6,0,3,3,6,7,2,7]

Your code is saved on this device.Solution